Jupiter is the fifth planet from the Sun and by far the largest — a gas giant with more than twice the combined mass of every other planet in the Solar System. Its composition of roughly 90% hydrogen and 10% helium (by number) mirrors the Sun itself, earning it the informal nickname of a “failed star.”

Jupiter’s atmosphere is organized into dark belts and light zones of east-west jet streams, with embedded vortices — most famously the Great Red Spot, an anticyclonic storm wider than Earth that has persisted for at least 190 years. Beneath the clouds, pressure and temperature rise until hydrogen transitions into a metallic, liquid state; this vast electrically conductive layer drives Jupiter’s enormous magnetic field — 20,000× stronger than Earth’s at the poles.

Jupiter’s 95 confirmed moons include the four Galilean satellites — Io, Europa, Ganymede and Callisto — each a world in its own right. Io is the most volcanically active body in the Solar System; Europa hides a global liquid-water ocean beneath its ice crust; Ganymede is larger than Mercury; Callisto is the most heavily cratered body known. The Juno spacecraft (2016-present) continues to map Jupiter’s gravity, magnetic field, and deep atmosphere; ESA’s JUICE (arriving 2031) and NASA’s Europa Clipper (arriving 2030) will follow up on its icy moons.